Electron beam lithography system having improved electron gun

ABSTRACT

An electron beam lithography system has an electron gun including at least one laser that is operable in a first mode to generate electrons for lithography. The electron beam lithography system is operable in a second mode to regenerate the photocathode of the electron gun by application of the laser. The photocathode includes a layer of cesium telluride.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority from U.S. ProvisionalApplication Serial No. 60/265,272, filed Jan. 31, 2001, which is herebyincorporated by reference in its entirety as if fully set forth herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to electron beam lithography and,in particular, an electron beam lithography system having an improvedelectron gun.

[0004] 2. Description of the Related Art

[0005] The greatest limitation on electron beam lithography systems hasbeen their speed of operation, or throughput. Multiple electron beamtechnology holds the potential for greatly improving throughput in thenext generation (sub 100 nm) of lithography. In this approach, multipleelectron beams are formed by focusing an array of independentlymodulated light (laser) beams onto a photocathode in transmission mode.Electron beams are emitted if the energy of the photon beam is greaterthan the work function of the photocathode material. Once created, theelectron beams are accelerated, focused and scanned across the wafer ormask using an electron-optical column. In such systems, the selection ofthe photocathode material is an important performance-limiting factor.

[0006] For an electron gun employing a photocathode source, the totalcurrent that can be delivered to the wafer of substrate is limited bythe quantum efficiency (QE) of the photocathode and the transmission ofthe electron column. The QE is the ratio of the number of emittedelectrons to the number of incident photons and is largely an intrinsicproperty of the material. The column transmission is related to theenergy distribution of the emitted electrons, such that a wider energydistribution results in a lower column transmission.

[0007] Many photocathode materials have been used. These includemetallic films such as gold; semiconductor materials such as galliumarsenide; and semiconductor materials whose surfaces have been treatedwith cesium and oxygen, referred to as negative electron affinity (NEA)photocathodes.

[0008] Generally, the highest quantum efficiencies are produced with NEAphotocathodes (as large as 30% with an energy of 100 meV). However, theQE of NEA photocathodes is very sensitive to vacuum contaminants (e.g.,a vacuum of 10⁻¹¹ Torr is generally required to maintain a high QE).Further, the QE of NEA photocathodes degrades with time as current isemitted from the surface. This degradation is caused by a loss of cesiumfrom the surface due to desorption.

[0009] Gold photocathodes, on the other hand, are much less sensitive tovacuum conditions and can operate under high vacuum conditions. However,gold photocathodes typically exhibit low QE, which ultimately wouldresult in low throughput.

[0010] There is therefore a need for an electron gun having aphotocathode with relatively high quantum efficiency and that canoperate at relatively high vacuum. There is a further need for anelectron beam lithography system having such an electron gun.

SUMMARY OF THE INVENTION

[0011] These and other drawbacks in the prior art are overcome in largepart by a system and method according to the present invention.

[0012] An electron beam lithography system according to animplementation of the invention has an electron gun including a layer ofcesium telluride. The electron beam lithography system further includesat least one laser that is operable in a first mode to generateelectrons for lithography. The electron beam lithography system isoperable in a second mode to regenerate the photocathode of the electrongun by application of the laser.

[0013] An electron gun according to an implementation of the inventionincludes at least one laser and a photocathode having a layer of cesiumtelluride. The photocathode may be applied on a substrate or may have ametallic layer interposed between the cesium telluride layer and thesubstrate. Electrodes may be applied to the cesium telluride layer toapply current to regenerate the photocathode.

[0014] A method according to an implementation of the invention includesapplying a laser to a photocathode in a first mode to emit electrons andin a second mode to regenerate the photocathode. In one implementation,the laser is applied at a power density of approximately 10⁴ Watts persquare centimeter in the first mode and at a power density in the rangesubstantially comprising 10⁴-10⁶ Watts per square centimeter in thesecond mode. Temperature of the photocathode in the second mode israised to approximately 20-200 degrees Celsius above room temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] A better understanding of the invention is obtained when thefollowing detailed description is considered in conjunction with thefollowing drawings in which:

[0016]FIG. 1 is a block diagram of an exemplary electron beamlithography system in accordance with an implementation of theinvention;

[0017]FIG. 2 is a more detailed diagram of the laser optics of FIG. 1;

[0018]FIG. 3 and FIG. 4 illustrate electron optics for the system ofFIG. 1;

[0019]FIGS. 5A-5C illustrate exemplary photocathodes according toembodiments of the present invention;

[0020]FIG. 6 is a graph of QE vs. power density for a photocathodeaccording to an implementation of the invention; and

[0021]FIG. 7 illustrates a process according to an implementation of theinvention;

[0022]FIG. 8 and FIG. 9 illustrate an interlaced writing strategyaccording to an embodiment of the present invention;

[0023]FIG. 10 is a simplified optical diagram of a lithography columnaccording to an embodiment of the present invention; and

[0024]FIG. 11 is a simplified diagram of a lithography tool.

DETAILED DESCRIPTION-OF THE INVENTION

[0025] System Overview

[0026] Turning now to FIG. 1, a block diagram of an electron beamlithography system according to an implementation of the presentinvention is shown and generally identified by the reference numeral100. As shown, the system 100 includes laser optics 102, electron optics104, and an electronics datapath 106, which receives an input 108.

[0027] The laser optics 102 include a laser 110, beam splitter 112, oneor more modulators 114, such as acousto-optical modulators, and anoptical system 116. In one implementation, the laser 110 is a 257 nmargon-ion laser. The electron optics 104 include a photocathode 118, anelectron beam column 120, and a writing plane 122, as will be explainedin greater detail below.

[0028] The datapath 106 includes a rasterizer 125, which produces asampled image 126, corrections unit 128, sequencer 130 and driveelectronics 132. An exemplary rasterizer is described incommonly-assigned U.S. Pat. No. 5,533,170, titled “Rasterizer for aPattern Generation Apparatus,” which is hereby incorporated by referencein its entirety as if fully set forth herein.

[0029] In operation, a laser beam 111, generated by the laser 110, issplit into a plurality of individual laser beams using the beam splitter112. In one implementation, the laser beam 111 is split into 32 beams bythe beam splitter 112. The array of individual laser beams enters thearray of acousto-optic modulators (AOM) 114, which switch on or off orset the transmitted photon flux of each individual beam to apredetermined value. In one implementation, thirty-two (32) AOMs areprovided.

[0030] The data needed to drive the AOMs are provided from the datapath106 and, particularly, the rasterizer 125. That is, the switching of theAOMs 114 is controlled by a modulation signal 124, provided from thedrive electronics 132 of the datapath 106. The modulation signal isdetermined by the pattern to be exposed on the substrate and generatedby the datapath 106.

[0031] Beam blanking and modulation is implemented electronically at theAOM array level, which significantly simplifies the design of theelectron lithography system. The resulting shorter column minimizes theelectron-electron interactions and maximizes achievable throughput. Inthe AOM array 114, the modulation of the photon intensity is achieved byapplying RF power to the individual AOM channels. Applying differentlevels of RF power can be used for fine modulation of the photonintensity. A multiple gray level, multiple pass writing strategy may beused for this electron lithography system. Further, another AOM (notshown) may be inserted in the optical system upstream of the splitter toact as a fast auxiliary blanker. This additional AOM may be used duringscan retrace when additional blanking is needed but the shutter 208 a(FIG. 2) is too slow.

[0032] Individual photon beams 115, generated by the AOM array 114, aredemagnified by the optical system 116. The optical system 116 may beimplemented as one or more optical lenses focused on the photoemittingsurface of a photocathode 118.

[0033] Examples of a suitable photocathode are CS₂Te (cesium telluride)photocathodes, Mg (magnesium), negative electron affinity photocathodes,based for example on cesiated GaAs (gallium arsenide), cesiated GaN(gallium nitride), or silicon-cesium oxide nanoclusters and possiblygold with a covering of hydrocarbons. In operation, photons absorbed inthe photocathode layer 118 excite electrons above the vacuum level, anda portion of the electrons which do not lose enough energy (whilescattering in the photocathode layer itself) are emitted into vacuum.When a voltage (up to 50 kV) is applied to the extraction electrode, thephotoelectrons are accelerated and focused to form a multibeam pattern,i.e. a virtual image of the photocathode surface, which is a demagnifiedphotoemission image of the laser beam array. The electron beam column120 then demagnifies the multibeam pattern and scans it across thewriting plane 122.

[0034] Throughput Considerations

[0035] System throughput may be an important factor in a multisourcesystem. A first factor affecting throughput is the total current neededto pattern a substrate. A certain,fraction of the electron-sensitiveresist must be exposed. To a first approximation, this exposure requiresa maximum available electron dose, which can be calculated for a resistof given sensitivity. The throughput is determined by the time requiredto deliver this dose, which is proportional to the maximum totalelectron current. This total current is proportional to the number ofbeamlets N_(b) and the current I_(b) delivered by each beamlet. Thus thetime T to expose a given area is T=AS/N_(b)I_(b), where A is the area tobe patterned and S is the resist sensitivity (charge density required toexpose the resist). High throughput can be achieved by using asufficiently large number of beamlets and a sufficiently large currentin each beamlet.

[0036] A second factor affecting throughput is pixel delivery rate. Tocompose the pattern properly, the pixel spacing d must be not much morethan half the size of the smallest pattern features. The total number ofpixels, Σn, will be proportional to A/d², so the time required to writethe pattern is T=A/f N_(b)d², where f is the maximum beam incrementingrate. This time can be reduced by using a large number of beams and ahigh modulation rate. Because of the large number of pixels involved,the imodulation rate required for a single beam system quickly exceedsthe commonly accepted state of the art (300-500 MHz). For a giventhroughput, multiple beam approaches reduce the modulation rate by thenumber of beamlets used.

[0037] A third factor affecting throughput is that the stageacceleration and velocity must be matched to the electronic scan length.As the array of beams is scanned across the substrate, the area undereach electronic scan is given by LN_(b)d, where L is the length of theelectronic scan. The number of scans n needed to cover the entire area Ais therefore n=A/(LN_(b)d). In a write-on-the-fly scheme with acontinuously moving stage, the time T _(s) to travel from one scan lineto the next is T _(s)=N_(b)d/v, where v is the stage velocity. The timerequired to cover an entire substrate (ignoring all overheads such asacceleration, deceleration, and retrace times) is T=T _(s)n=A/Lv, wherethe larger scan length L allows for increased throughput.

[0038] For a high-throughput multisource column, a large array ofbeamlets with a large current in each beamlet, a high modulationfrequency, and a large scan field are desirable. For direct-writeapplications, a throughput of 10 (300 mm) wafers/hour may be achievablewith a resist sensitivity of 10 μC/cm² and a 50 keV multisourcelithography column with a total beam current of 16 μA. The array wouldcontain 200 beamlets individually switching at 300 MHz and scanned overa field of 1 mm, while the stage moves at a speed of 15 cm/sec. For maskpatterning applications, where the throughput is reduced to one 9 inchmask/6 hours, a column with 32 beams, total beam current of less than 1μA, scan field of 0.6 mm, blanking rate of 100 MHz and stage speed ofless than 1 cm/sec are sufficient.

[0039] Laser Optics

[0040] A more detailed diagram of the laser optics 102 (FIG. 1) is shownin FIG. 2. The beam 111 from the 257 nm laser is actively controlled byautomatic beam centering mirrors 202 so that alignment to the opticaltrain, both in position and angle, is maintained. An attenuator 204, acombination polarization rotating element and polarizing beam splitter,adjusts the laser power to a range suitable for operation of the systemwhile allowing the laser to operate in a power range optimized forreliability and light noise control. A spatial filter 205 removes thelobes from the sine squared intensity profile caused by the frequencydoubling crystal inside the laser. Anamorphic relay optics 206 create around beam from the radiation exiting this aperture and relays it to adiffractive optical element (DOE) 209 inside the brush module 210.

[0041] The DOE 209 is a grating that produce 32 beams that are focusedby the lenses inside the brush module to waists under the transducers ofthe multi-channel AOM 114. The AOM 114 diffracts part of the light fromeach beam in response to the RF signal applied to each channel. Theintensity of the diffracted beam can be adjusted by the power level ofthe RF signal thus allowing for separate gray scale modulation of eachchannel.

[0042] A mechanical shutter 208 before a brush module 210 is used toblock all light from reaching the photocathode when the system is notwriting. A blanker AOM 208 a may be provided for blanking during scanretrace. A K-mirror 212 allows for rotational adjustment of the lineararray of beams exiting the AOM. A wave plate 214 aligns the polarizationof the beams for optimal focusing through birefringent photocathodesubstrates such as sapphire. A lens element 215 after the wave plate 114focuses the array onto an afocal spot on the steering mirror. Beforereaching the steering mirror, the zero order or undiffracted, light fromthe AOM is blocked by the 0 order beam stop 216. A steering mirror 218allows for small positional adjustment of the spot array at the finalimage plane of the objective. The zoom optics and stigmator 220 relaythe focal spot into the pupil of the objective lens 222. Tilted platesinside the stigmator provide adjustment capability to ensure that thefocus of the spots on the photocathode substrate occurs in the sameplane whether measured along the direction of the array of spots orperpendicular to it. Movable lenses within the zoom allow for slightmagnification adjustment of the array. According to one implementationof the present invention, the objective 222 has a NA of 0.57 andpresents a 300 nm FWHM spot to the photocathode material.

[0043] Table 1 shows how the array of 32 beams is demagnified throughoutthe system. The 42 um spot size inside the AOM 114 gives a 7 ns soundtransit time in the fused silica interaction medium. A 300 MHz carrierfrequency is used to diffract the beams with a 10 nsec pixel time. The42 um spot size in the AOM is reduced to 300 nm at the photocathode bythe optical train. This includes spot enlargement from 225 nm to 300 nmcaused by aberrations in the optical train. This spot size is furtherreduced by the electron optics to 50 or 70 nm depending on the electroncolumn demagnification. This also contains some allowance for spotenlargement from e-beam aberrations. TABLE 1 Brush parameters throughthe optical and e-beam sub-systems. All spot sizes are full width halfmaximum. At Plate At Photocathode At AOM Low res./ Low res./ Low res./Parameters high res. high res. high res. Unaberrated beam dia. 50 nm/30nm 225 nm 42 μm Aberrated beam dia. 70 nm/50 nm 300 nm 42 μm Beamseparation 350 nm/ 2.10 μm .392 mm 210 nm Brush width 10.85 μm/ 65.1 μm12.152 mm 6.51 μm Demag from previous 6.0/10 186.66 . . . NA stageStripe brush increment 1600 nm/ NA NA 960 nm Filled-in grid 25 nm/15 nmNA NA

[0044] Electron Optics

[0045] Exemplary electron optics are shown in greater detail in FIG. 3and FIG. 4. In operation, an array of laser beams exiting the AOM array114 (FIG. 1) is focused on the photoemissive layer of a planarphotocathode 118 (FIG. 1, FIG. 4). The photocathode 118 is placed in astrong extraction field, typically 5-10 kV/mm in order to minimize axialaberrations.

[0046] In the embodiment illustrated, the photocathode 118 is biased at−50 kV, and is separated from a grounded extraction electrode (anode)304 by an accelerating gap a (FIG. 4). The extraction electrode 304 istypically a planar electrode with an aperture of diameter d in thecenter. The accelerating field forms a 1× magnified virtual image 311 ofthe photocathode surface at a distance a above the photocathode surface,and the divergent lens action of the aperture forms a demagnified (⅔×)virtual image 313 at a distance a/3 above the photocathode.Simultaneously, this aperture lens shifts the plane of the virtualsource of illumination to a plane spaced a distance of 3a above thephotocathode 118.

[0047] Near the optical axis 301, the resolution of the virtual imageformed by the accelerating field is limited by the spherical andchromatic aberration. For emission sites at the photocathode furtheraway from the optical axis, the electron-optical image can suffer fromoff-axis aberrations, which can severely limit the available field sizeat the photocathode and therefore limit the maximum array size. A largephotoemission area allows increased separation between beamlets, whichreduces the effect of electron-electron interactions. For such largephotoemission areas, an additional electron (field) lens 306, in closeproximity to the photocathode 118, is used in order to minimize off-axisaberrations in the following demagnification lenses. Further details ofexemplary electron optics are shown in co-pending U.S. patentapplication Ser. No. ______, titled A COMPACT PHOTOEMISSION SOURCE,FIELD AND OBJECTIVE LENS ARRANGEMENT FOR HIGH THROUGHPUT ELECTRON BEAMLITHOGRAPHY, and WO055690A2, which is hereby incorporated by referencein its entirety as if fully set forth herein. The field lens 306collimates the electrons exiting the accelerating region and forms acrossover in the plane of the beam-limiting aperture. The virtual imagecreated by the field lens is then subsequently demagnified by thedemagnification and objective (magnetic) lenses 310, 314 (FIG. 3) toform a array of focused beams. A set of alignment coils 309 is used tocenter and stigmate the electron beam array in the beam-limitingaperture and in the objective lens.

[0048] According to one implementation of the invention, a set ofmultiple stage deflection coils 312 is used to scan the array ofindividually blanked beamlets across the substrate, and another set ofdeflection coils performs dynamic stigmation and focus as the array isscanned across the full field. This allows dynamic stigmation and focuscorrections to be applied to different parts of the scan field. X and ydeflection corrections can also be added to different parts of the scanfield

[0049] The beams inside the AOM 114 must be spread out so that there isno optical interference or acoustic crosstalk between them. However, thefinal integrated image in resist must be composed of overlapping spots.This is accomplished by employing an interlaced scan print strategy andwriting with multiple passes, as will be described in greater detailbelow.

[0050] Electron Optics—Photocathode

[0051] Another important factor to be considered in the optical designis the electron emission properties of the photocathode. Theseproperties need to be closely matched to the electron-optical parametersof the column. Key photoemission parameters, i.e., source size, angularand energy distributions of photoemitted electrons, and photocathodequantum efficiency (QE), are critical for determining the columngeometry and related laser optical design parameters. Photocathodes withhigh quantum efficiency, low transverse energy spread, low noise, highstability, and long lifetime are desirable.

[0052] Electrons are emitted from the photocathode surface into vacuumin all directions, in a full solid half-angle with a specific angularand energy distribution.

[0053] When a voltage is applied to the extractor, the electrons areaccelerated and collimated by the extraction electrode into a narrowcone defined by the source half-angle α₀ and then focused by one or morelenses at the substrate plane, as shown in FIG. 10. The sourcehalf-angle α₀ determines the brightness of the source-and thereforeaffects the column design and system architecture of the tool.

[0054] The optimum image acceptance angle α_(i) determined by thetradeoff between objective lens aberrations and e-e (electron-electron)interactions, is typically 10 mrad or less. For smaller image acceptanceangles α_(i), the blur due to geometric and chromatic aberrationsdecreases, but e-e interactions increase. For image acceptance angleslarger than 10 mrad, the e-e aberrations decrease, however the geometricand chromatic aberrations can become excessive.

[0055] Assuming that the lenses have a demagnification M(M<1), theangular magnification M_(α)=1/M and therefore the maximum source angleα_(0max) that can be accepted by the column is α_(0max)=α_(i)/M_(α). Therequired demagnification ratio is set by the achievable photoemissionsource size, which is determined by the laser spot size and requiredbeam spot size at the substrate. For example, for a laser spot of 0.5 μmand a required beam spot size of 70 nm at the substrate, theaccelerating region demagnifies the spot by ⅔, and a demagnificationratio of 10×(M=0.1) is required when electron optical aberrations of theelectron lenses are included. At this demagnification ratio, and forα_(i)=10 mrad, the maximum accepted source angle α₀=1 mrad. When thesource angle α₀ is larger than α_(0max), a fraction of the beamletcurrent must be cut out by the beam limiting aperture, and only afraction T (here T can be thought of as a transmission coefficient),where T=(α_(0max)/α₀)², can be used for beam exposure.

[0056] In summary, for maximum efficiency, one can reduce themagnification M to increase the maximum source angle α_(0max) ordecrease the source angle α₀. Typically, α₀>α_(0max), which means thatsome percentage of photoemitted electrons are lost. The source angle α₀depends on the beam energy E, the energy spread, and angulardistribution of photoemitted electrons. For a given photocathode, thisangle varies with accelerating voltage as$\alpha_{0} = \sqrt{\frac{\Delta \quad E_{tr}}{E}}$

[0057] where ΔE_(tr) is the transverse energy spread. For a goldphotoemitter illuminated with a 257 nm laser, the energy spread isestimated to be about 0.5 eV, which corresponds to a source angle α₀ of3.2 mrad at 50 keV. The transmission coefficient T is then 0.1, i.e.,only 10% of the generated photoelectrons can be used to expose thesubstrate. This means that significantly more laser power is needed togenerate the current required at the substrate. Ideally, one would liketo minimize the transverse energy spread and simultaneously maximize thequantum efficiency. The laser power P needed to generate the requiredsource current I₀ equals P=I₀/η, where η is the quantum efficiency,which near the threshold in Fowler's approximation is proportional to

η∝(hv−Φ)²∝(ΔE)²

[0058] where hv is the photon energy, Φ is the work function of thephotocathode, and ΔE is the energy spread. For a given laser photonenergy hv and work function Φ, the requirement for a maximumphotocurrent and minimum energy spread can be restated as finding aphotocathode with the steepest (largest slope) dependence of quantumefficiency upon photon energy.

[0059] The optical parameters of the laser, electron optics, and sourceare closely related. Assume a simplified column (FIG. 10), where thelaser generated source has a diameter of d₀ and total source current ofI₀. The photoemifted electrons are emitted into a solid angle α₀. At thesubstrate, an image current I_(i), and a beam diameter d_(i), arerequired to meet the lithography print quality and throughput. Themagnification M of the column is defined as M=d_(i)/d₀, i.e.,M_(α)=d₀/d_(i)=α_(i)/α_(0max) and the source and image currents arerelated by I_(i),=TI₀. When aberrations are neglected, the brightnessβ_(i) is conserved in the imaging process. The source current density J₀at the photocathode can be written as J₀=J_(i)M²/T, where M is smallerthan 1. J₀ is a constant because M²/T=(α_(i)/α₀)² is a constant for agiven optimized objective lens and particular photocathode material, andJ_(i), is determined by throughput requirements. Assuming thatlithographic print quality and throughput require a spot size of 50 nmand beamlet current of 20 nA, a current density of about 1000 A/cm² willbe needed at the substrate.

[0060] For an image acceptance angle of 10 mrad, the required beambrightness at the substrate is >3×10⁶ A/cm²sr, and therefore aphotocathode brightness of about 10⁷ A/cm²sr is desirable. For ademagnification of M=0.1 and a column transmission T=0.1, a currentdensity of about 100 A/cm² must be delivered by the photocathode. Therequired high current density can cause significant photocathodedegradation due to electron- and photon-stimulated desorption andchemistry at the photocathode surface and surrounding surfaces.

[0061] For conventional metal photocathodes, i.e., gold, silver, etc.,the quantum yield and photoyield (nA/mW) are quite low even for UVlight. A stable, high power UV laser is needed to provide sufficientbeam current. For example, for a gold photoemitter with a photoyield of10 nA/mW, a total power of 20 mW is needed to generate a beamlet currentof 200 nA at the source, which is then reduced by the beam-limitingaperture to a current of 20 nA/beamlet at the substrate.

[0062] For 32 beams, a total laser power of 640 mW is needed, which hasbecome recently available in the most powerful UV lasers. When the laserpower utilized in each beamlet is focused into a 0.5 μm spot, a laserpower density of ˜2×10⁷ W/cm² is reached at the photocathode surface.The required high laser power and small spot sizes result in very highpower densities, which can cause significant photoyield degradation dueto photon-induced surface effects, i.e., photodesorption andphoton-stimulated surface chemistry.

[0063] In addition, a significant increase in temperature can beexpected for low thermal conductivity substrates (quartz, fused silica),because a substantial amount (>70%) of power is being dissipated in thethin photoemissive layer. For continuous laser illumination with a laserpower P focused in a spot diameter d on a substrate with thermalconductivity κ, the temperature rise is proportional to P/κd. Here weneglect the heat conduction through the photoemissive metal layer due toits small thickness (˜15 nm). For example, using a fused silicasubstrate with a thermal conductivity of 0.014 W/cm K and an absorbedlaser power of 20 mW focused into a 0.5 μm spot, the temperature rises˜4200 K at the location of the focused laser spot. This temperature riseis high enough to cause local melting of the gold layer, inducemorphological changes, and alter the surface of the photocathode.

[0064] One possibility for increasing the thermal dissipation capacityof the cathode is to use a higher thermally conductive substrate, e.g.,sapphire, which is readily available and has a thermal conductivity of0.36 W/cm K; this would result in a 26 times smaller temperatureincrease. However, sapphire is birefringent and a specific choice ofcrystal orientation may be needed to achieve submicron spot sizes.Ultimately, a natural or CVD diamond substrate may be needed. Diamond isoptically clear in the UV range and has a thermal conductivity of 20W/cm K, which is more than 3 orders of magnitude greater than fusedsilica; this results in a negligible temperature rise.

[0065] Assuming that the source angle α₀ can be approximated as$\alpha_{0} = {\sqrt{\frac{\Delta \quad E_{tr}}{E}} \approx \sqrt{\frac{\Delta \quad E}{E}}}$

[0066] the required laser power P${P \propto \frac{I_{0}}{\left( {\Delta \quad E} \right)^{2}}} = {\beta_{i}\frac{\pi^{2}d_{0}^{2}}{4\left( {\Delta \quad E} \right)E}}$

[0067] can be minimized through a careful choice of electron-opticalparameters.

[0068] A significant reduction of the laser power can be achieved byemploying higher quantum efficiency cathodes, i.e., magnesium or Cs₂Teor GaAs based negative electron affinity (NEA) photocathodes, thoughboth may require relatively high vacuums and periodic recesiation of thephotocathode surface.

[0069] In summary, basic conclusions can be drawn for the column design.Lithographic print quality and throughput requirements determine thebrightness. The image angle a, could be theoretically increased;however, due to geometric and chromatic aberrations of the objectivelens, it is expected to reach no more than 10 mrad. An increase in beamenergy E above 50 keV would decrease the required laser power, butincreased substrate heating and reduced resist sensitivity would becomeproblems. An increase in energy spread ΔE, due to a lower work functionhv or larger photon energy Φ, will reduce the required laser power dueto the increase in quantum efficiency. Nevertheless, the energy spreadshould not increase above ˜1 eV, because the chromatic aberration andtherefore the beam spot size at the substrate increases.

[0070] Additionally, Fowler's approximation becomes inaccurate for largeΔE, resulting in a smaller than expected increase in quantum efficiency.Due to its quadratic dependence, the most effective way to reduce thelaser power is to decrease the laser spot size d₀. A smaller laser spotsize allows a smaller demagnification ratio, which minimizes thefraction of the electron beam cut by the aperture. Smaller spot sizescan be achieved by utilizing shorter wavelength lasers or by patterningthe photocathode, therefore confining the emission to a smaller spot. Inthe optimum case, when the brightness of the source matches the beambrightness required at the substrate, all electrons pass through thecolumn and the source is self-aperturing.

[0071] Electron Optics—Cesium Telluride Photocathode

[0072] As noted above, the photocathode 118 may be implemented as acesium telluride photocathode. In particular, the photocathode mayinclude a cesium telluride layer 500 (FIG. 5A-FIG. 5C), as will beexplained in greater detail below. Certain embodiments may also includea cesium bromide layer (e.g., 5-10 nm) applied to the top of the cesiumtelluride layer. Cesium telluride (Cs₂Te) is a compound semiconductorwith a bandgap of 3.3 eV and an electron affinity of 0.2 eV. It canproduce a high QE (about 10%) when irradiated with 5 eV photons (240 nm)and has an energy spread of about 1.5 eV. In typical operation, photonsin the wavelength range of about 200 to about 300 nm are applied.

[0073] Exemplary cesium telluride photocathodes for use in an electrongun according to embodiments of the present invention are shown in FIGS.5A-5C. The photocathode 118 a of FIG. 5A includes a cesium telluridelayer 500 a grown on a transparent substrate 502 a such as sapphire. Aphotocathode 118 b as shown in FIG. 5B includes a cesium telluride layer500 b, a substrate 502 b, and a metallic layer 504 b interposed betweenthe cesium telluride layer and the substrate. The metallic layer 504 bmay be implemented as any semi-transparent metal, such as molybdenum,titanium, or platinum. In this case, current is carried by the metallayer up to the emission sites.

[0074] One advantage of using cesium telluride as-a photocathodematerial is that its QE (quantum efficiency) is relatively insensitiveto vacuum contaminants. However, over extended periods, the QE of cesiumtelluride has been shown to degrade due to exposure to background gases.This degradation may be reversed in a variety of ways.

[0075] According to one such method, the cesium telluride is heated, forexample, to 100 degrees Celsius by applying a current in the plane ofthe film. Such an embodiment of a photocathode 118 c is shown in FIG.5C. The photocathode 118 c includes a cesium telluride layer 500 c, ametal layer 504 c, and a substrate 502 c. In addition, contacts 506 a,506 b are provided, to apply a current/to the plane of the film and heatthe film during one or more regeneration cycles, thereby recovering QEdegradation.

[0076] However, this method may be disadvantageous in that it requiresthe additional electrodes to supply the current. One aspect of thepresent invention, therefore, is the recovery of QE by exposing thephotocathode to an intense electromagnetic beam, such as the exposinglaser itself, typically operating in the ultraviolet range. As shown inFIG. 6, a cesium telluride photocathode can be exposed to a powerdensity of 10⁷ Watts per square centimeters and still maintain a QEabove 4%. Moreover, because the QE of the cesium telluride photocathodeactually increases with power density, at least below a certainthreshold, the photocathode can be regenerated using the laser itself.Thus, a regeneration cycle may be provided whereby the laser that isused to cause electron emission in the photocathode is used to alsoregenerate the photocathode.

[0077] This process is shown with reference to FIG. 7. As shown, alithography cycle 702 is implemented under control of the controller(FIG. 1). According to one implementation of the invention, if the QE ofthe photocathode is 10%, this is made to occur at a power density ofapproximately 10⁴ Watts per square centimeter. Once QE has degraded to apredetermined degree, a regeneration cycle 704 is implemented. Accordingto one implementation, the regeneration cycle is done at a power densityof 10⁴-10⁶ Watts per square centimeter at a wavelength of approximately257 nanometers. The substrate temperature is raised about 20-200 degreesCelsius above room temperature.

[0078] Writing Strategy

[0079] As noted above, an aspect of the present invention is an improvedwriting strategy. FIG. 8 and FIG. 9 illustrate the interlaced scanstrategy. Shown in FIG. 8 is a portion of the leading edge of the brush.More particularly, the first five (5) beams 802 a-802 e of the brush areshown for clarity. The beams 802 a-802 e are separated by 350 nm in thelow magnification case and by 210 nm in the high magnification case. Thebrush is scanned in the direction perpendicular to the array-801. By thenext scan, the stage has moved by 1600 nm in the low magnification caseand by 960 nm in the high magnification case.

[0080] More particularly, FIG. 9 illustrates beam interlacing in greaterdetail. Shown are a plurality of offset brush lines 902 a-902 h and thesingle continuous line 900 formed therefrom. The offset brush settings902 a-902 h fill in the continuous line in the Y direction while forminglines in the x direction. The numbers in Table 1 represent spacingsbetween the beams in the y direction.

[0081] As shown in the table, after 6 scans a contiguous region of scanlines separated by 50 nm begins to be filled in. Table 2 shows theseparation between adjacent scan lines after 6 scans. As more scans areadded this uniform region grows in extent. TABLE 2 7, 7, 4, 3, 1, 3, 4,3, 4, 3, 4, 3, 1, 3, 3, 1, 3, 3, 1, 3, 3, 1, 3, 3, 1, 3, 1, 2, 1, 3, 1,2, 1, 3, 1, 2, 1, 3, 1, 2, 1, 3, 1, 2, 1, 1, 2, 1, 2, 1, 1, 2, 1, 2, 1,1, 2, 1, 2, 1, 1, 2, 1, 2, 1, 1, 2, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 2,1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1,1, 2, 1, 2, 1, 1, 2, 1, 2, 1, 1, 1, 2, 1, 2, 1, 1, 2, 1, 3, 1, 2, 1, 3,1, 2, 1, 3, 3, 1, 2, 1, 3, 1, 3, 3, 1, 3, 3, 1, 3, 3, 1, 3, 3, 1, 3, 4,3, 4, 3, 4, 3, 4, 3, 4, 7, 7, 7, 7

[0082] There are only selected combinations of number of beams, beamseparation, and stripe brush increment that will yield a uniformlyfilled in region. The examples of brush parameters given in Table 1 areexamples but are not completely unique. The variable demagnification andfocus capability in the electron column could be used to make a brushthat minimizes e-e position errors for a given throughput.

[0083] Another method for interlaced scanning is described in greaterdetail in co-pending U.S. patent application Ser. No. ______, titled“Laser Pattern Generator,” which is hereby incorporated by reference inits entirety as if fully set forth herein. Another feature of thescanning strategy is to arrange the direction of the sound field in theAOM, as imaged on the writing surface, to be in the opposite directionas the scan velocity. This improves the sharpness of the modulatededges.

[0084] Beam Alignment

[0085] Aligning the light-optical system to the electron-optical systemis challenging. The optical axis of the final light optical reductionlens should be perpendicular to the photocathode and the separationbetween the two set to high accuracy. This lens must also be aligned tothe axis of the first lens of the electron optical column and to theincident light beams. The incident light beams should also beperpendicular to the photocathode.

[0086] One method for aligning the light optical system to the opticaxis of the column is shown with reference to FIG. 11. Moreparticularly, shown in FIG. 11 is a simplified diagram of a photocathodedriven multiple beam lithography tool. The tool includes an illuminationsource 1100, a light conditioning apparatus 1102, mirrors 1104, 1106, alight optical reduction lens(es) 1108, a photocathode 1110,electron-optical lenses 1102, and a mask or wafer substrate 1114.

[0087] In operation, the light from the illumination source 1100 issplit into multiple beams by the light conditioning apparatus 1102(e.g., one or more acousto-optical modulators). The light conditioningapparatus 1102 also varies the intensity of each beam and individuallyblanks the beams, as described above, thereby creating an array ofindividually blanked and intensity modulated beams. It is noted,however, that other methods may be used to generate this array, such asa separate laser diode for each beam, controlled individually.

[0088] The mirrors 1104, 1106 direct the array into the reduction lens1108. Position adjustments on the mirrors 1104, 1106 allow the array tobe shifted in position and angle with respect to the reduction lens1108. The photocathode 1110 is maintained at a relatively high negativevoltage. The reduction lens 1108 is mounted relatively close to thecathode 1110, making electrical isolation problematic. Consequently, itis often advantageous to connect the reduction lens 1108 to the cathodepotential. However, this can cause problems in alignment since thereduction lens must be moved while at high voltage.

[0089] As such, the following alignment scheme may be used to align thecolumn: First, the reduction lens 1108 is removed and the “raw” beam isaligned to the electron-optical axis 1116 of the column. This is done bywobbling the first lens in the column (oscillating the lens strengthabove and below focus) and moving the position of the light beam on thephotocathode until the defocus of the electron beam is greater than itsshift in position. This position is noted for later comparison using theimaging capability of the column.

[0090] Second, the incident beam is adjusted so that it is normal to thesurface of the photocathode 1110 while still impinging on thephotocathode 1110 at the same position. This is done by adjusting themirrors 1104, 1106 until the separation of the reflection from thephotocathode 1110 and the incident beam at some specified distance fromthe photocathode 1110 is smaller than a specified distance. These twodistances may be used to calculate the angle between the incident andreflected beams. It is noted that these two steps may be reversed.

[0091] Finally, the reduction lens 1108 is re-inserted and its positionand angle are adjusted until the beam reflected from the lens is alignedto the incident beam and the spot on the cathode 1110 is coincident withthe position determined in the first step. If the required angleaccuracy cannot be obtained using reflections from the lens, an opticalflat may be placed on the lens barrel to aid in this alignment.Alignment of the demagnified light beam may be further refined by movingthe spot on the cathode while wobbling the first electron lens, thenrepeating the second step.

[0092] The invention described in the above detailed description is notintended to be limited to the specific form set forth herein, but isintended to cover such alternatives, modifications and equivalents ascan reasonably be included within the spirit and scope of the appendedclaims.

1. An electron beam lithography system, comprising: an electron gun,said electron gun comprising: at least one laser; and a photocathodesubstantially comprising cesium telluride and adapted to be activated togenerate electrons by said at least one laser and to be regenerated byexposure of at least one surface of said photocathode to radiationgenerated from said at least one laser.
 2. An electron beam lithographysystem in accordance with claim 1, wherein said photocathode comprises acesium telluride film on a substrate.
 3. An electron beam lithographysystem in accordance with claim 2, wherein said photocathode includes ametallic film interposed between said cesium telluride layer and saidsubstrate.
 4. An electron beam lithography system in accordance withclaim 2, including means for applying a current in a plane of saidcesium telluride layer.
 5. A method for electron beam lithography,comprising: applying at least one laser in a first mode to a cesiumtelluride photocathode for generating electrons; and applying said atleast one laser to said cesium telluride photocathode in a second modeto regenerate said cesium telluride photocathode.
 6. A method accordingto claim 5, wherein in said first mode, said laser is applied at a powerdensity of approximately 10⁴ Watts per square centimeter.
 7. A method inaccordance with claim 6, wherein in said second mode, said at least onelaser is applied at a power density ranging from about 10⁴-10⁶ Watts persquare centimeter.
 8. A method in accordance with claim 6, wherein insaid second mode, said at least one laser is applied to at least onesurface of said photocathode for a time sufficient to raise atemperature of said cesium telluride to a temperature ranging from about20° C. to about 200° C. above room temperature.
 9. A method inaccordance with claim 26, wherein a wavelength of said laser comprisesapproximately 257 nanometers.
 10. An electron gun, comprising: at leastone laser; and a photocathode adapted to be activated to generateelectrons by said at least one laser and to be regenerated by exposureof at least one surface of a photocathode to radiation generated fromsaid at least one laser
 11. An electron gun in accordance with claim 10,wherein said photocathode comprises a cesium telluride film on asubstrate.
 12. An electron gun in accordance with claim 11, saidphotocathode including a metallic film interposed between said cesiumtelluride layer and said substrate.
 13. A method, comprising: providingat least one laser; and providing a photocathode adapted to be activatedto generate electrons by said at least one laser and to be regeneratedby exposure of at least one surface of said photocathode to radiationgenerated from said at least one laser.
 14. An method in accordance withclaim 13, wherein said photocathode comprises a cesium telluride film ona substrate.
 15. An method in accordance with claim 14, wherein saidphotocathode includes a metallic film interposed between said cesiumtelluride layer and said substrate.
 16. An electron beam lithographysystem, comprising: an electron column; and an electron gun; whereinsaid electron gun is adapted to apply at least one laser in a first modeto a cesium telluride photocathode for generating electrons; and saidelectron gun is adapted to apply said at least one laser to at least onesurface of said cesium telluride photocathode in a second mode toregenerate said cesium telluride photocathode.
 17. An electron beamlithography system according to claim 16, wherein in said first mode,said at least one laser is applied at a power density of approximately10⁴ Watts per square centimeter.
 18. An electron beam lithography systemin accordance with claim 16, wherein in said second mode, said at leastone laser is applied at least one surface of said photocathode at apower density 10⁴-10⁶ Watts per square centimeter.
 19. An electron beamlithography system in accordance with claim 16, wherein in said secondmode, said at least one laser is applied to at least one surface of saidphotocathode for a time sufficient to raise a temperature of said cesiumtelluride photocathode to a temperature ranging from about 20° C. toabout 200° C. above room temperature.
 20. An electron beam lithographysystem in accordance with claim 27, wherein a wavelength of said lasercomprises approximately 257 nanometers.
 21. A controller for an electronbeam lithography system, said controller adapted to control applicationof at least one laser to a photocathode in a first mode for generatingelectrons and in a second mode for regenerating said photocathode.
 22. Acontroller in accordance with claim 21, said photocathode comprising acesium telluride photocathode.
 23. A controller according to claim 21,wherein said controller is adapted to control application of said atleast one laser in said first mode, such that said at least one laser isapplied at a power density of approximately 10⁴ Watts per squarecentimeter.
 24. A controller in accordance with claim 21, wherein saidcontroller is adapted to control application of said at least one laserin said second mode, such that said at least one laser is applied to atleast one surface of said photocathode at a power density ranging fromabout 10⁴-10⁶ Watts per square centimeter.
 25. A controller inaccordance with claim 21, wherein said controller is adapted to controlapplication of said at least one laser in said second mode, such thatsaid at least one laser is applied to at least one surface of saidphotocathode for a time sufficient to raise a temperature of said cesiumtelluride to a temperature ranging from about 20° to about 200° aboveroom temperature.
 26. A method in accordance with claim 8, wherein saidlaser generates an ultraviolet wavelength which is applied to saidsurface of said photocathode.
 27. A method in accordance with claim 19,wherein said laser generates an ultraviolet wavelength which is appliedto said surface of said photocathode.
 28. A method in accordance withclaim 2, where a cesium bromide layer overlies said cesium telluridefilm.
 29. A method in accordance with claim 11, wherein a cesium bromidelayer overlies said cesium telluride film.
 30. A method in accordancewith claim 14, wherein a cesium bromide layer overlies said cesiumtelluride film.